A flat panel display device (FPD), such as an organic EL (electroluminescence) display (OELD), a plasma display panel (PDP) or a liquid crystal display device (LCD), has such a structure that a glass substrate for an element having a light-emitting element formed and a glass substrate for sealing are disposed to face each other and the light-emitting element or the like is sealed in a glass package comprising two such glass substrates bonded. Further, for a solar cell, such as a dye-sensitized solar cell, application of a glass package having a solar cell element (dye-sensitized photoelectric conversion element) sealed with two glass substrates has been studied.
As a sealing material to seal a space between two glass substrates, a sealing resin or sealing glass has been used. Since an organic EL (OEL) element or the like is likely to undergo deterioration by water, the application of sealing glass excellent in moisture resistance, etc. is in progress. Since the sealing temperature of the sealing glass is at a level of 400 to 600° C., properties of the electronic element portion of the OEL element or the like will be deteriorated when firing is conducted by using a heating furnace. Accordingly, it has been attempted that a layer of a glass material for sealing containing a laser absorbent (sealing material layer) is disposed between sealing regions provided on the peripheral portions of two glass substrates, and the layer of the glass material for sealing is locally heated and melt to perform sealing by being irradiated with laser light (Patent Documents 1 to 3).
In order to carry out sealing by locally heating by use of laser light (laser sealing), vanadium glass (see Patent Document 1), bismuth glass or phosphate glass (see Patent Documents 2 and 3) or the like have been employed as the sealing glass (glass frit). While the laser sealing can suppress thermal influences on the electronic element portion, it is likely that residual stress is generated at the bonding interfaces between the glass substrates and a sealing layer formed by a melt-bonded layer of a sealing material layer since the laser sealing is a process for locally and quickly heating and cooling the sealing material layer. The generation of residue stress at the bonding interfaces causes cracks or fractures in the boding portion or the glass substrates, or a decrease in the bonding strength between the glass substrates and the bonding layer.
When the local heating and sealing operation, such as laser sealing, is applied to seal the space between the two glass substrates, it is likely that the local heating and sealing process generates residual stress at the bonding interfaces between the glass substrates and the sealing layer, which causes a problem in that it is impossible to sufficiently increase the bonding strength between the glass substrates and the sealing layer. This problem becomes a factor for reducing the reliability of FPDs, such as OELDs, PDPs and LCDs, or solar cells. The decrease in the bonding strength caused by the local heating and sealing process occurs not only in laser sealing but also in e.g. local heating by use of infrared light.